Ink printing apparatuses can be used for single-color or multicolor printing to a web-shaped printing medium, for example a paper web. The design of such ink printing apparatuses is known; see for example EP 0 788 882 B1. Ink printing apparatuses that, for example, operate according to the Drop-on Demand (DoD) principle have as a printer a print head or multiple print heads with nozzle units comprising ink channels and activators, wherein the activators—controlled by a printer controller—can excite ink droplets in the direction of the printing substrate, which ink droplets are directed onto the printing substrate in order to apply print dots for a print image there. The activators can generate ink droplets thermally (bubble jet) or piezoelectrically.
For example, the ink printing apparatuses may have print bars with respective multiple ink print heads. As viewed in the direction of movement of the recording medium, at least one dryer follows the print bars in order to dry the recording medium that has been printed to with the liquid ink. The heating capacity of the dryer is set such that, during the printing operation during which the recording medium is moved with continuous velocity, the recording medium is dried to such an extent that the moisture introduced with the ink is removed. If the operation of the ink printing apparatus is interrupted during a printing pause for example, then so much heat is stored in the region of the dryer and at transport rollers following the dryer, that a segment of the recording medium that is located in this region dries out more significantly than during printing operation.
In the unprinted state, the recording medium (paper, for example) has a defined basic moisture. Given an interruption of the printing operation, the danger exists that a large portion of this basic moisture is removed from the paper. Since, during a longer pause of the printing operation, the recording medium is significantly dried in the region of the dryer and in the region following the dryer (as viewed in the direction of movement of the recording medium), and in contrast to this retains its moisture in the remaining regions, transition regions arise in which segments of normal moisture and significantly dried segments abut one another. The significantly dried segments of the recording medium are somewhat contracted relative to the segments of normal moisture, whereby there is warping in these transition regions. This warping may form waves.
According to U.S. Pat. No. 6,837,635 B1, given a formation of waves in the recording medium in an inkjet printer it is known to keep the waves outside of the printing area so that the print heads may not be damaged.
According to JP 02122967 A, folds or waves in the recording medium are measured by means of an ultrasonic sensor. The height of the print heads is adjusted depending on the determined height of the recording medium. Here as well it is avoided that the print heads come into contact with the recording medium.